U.S. patent number 4,680,550 [Application Number 06/713,635] was granted by the patent office on 1987-07-14 for high-frequency antenna device in apparatus for nuclear spin tomography and method for operating this device.
This patent grant is currently assigned to Siemens Aktiengesellschaft. Invention is credited to Norbert Krause.
United States Patent |
4,680,550 |
Krause |
July 14, 1987 |
High-frequency antenna device in apparatus for nuclear spin
tomography and method for operating this device
Abstract
An antenna device for coupling to a magnetic high-frequency
field in apparatus for nuclear spin tomography, includes at least
two conductor sections of predetermined length which extend on an
imaginary cylinder surface parallel to the direction of the axis of
the imaginary cylinder and are arranged for current flow in
opposite directions when one end of each of the sections is
connected to an external device, a tubular enclosure arranged at a
predetermined distance concentrically with respect to the imaginary
cylinder surface surrounding the conductor sections, the enclosure
being at least largely permeable to low frequencies for magnetic
gradient fields, containing electrically highly conductive material
and adapted to be connected to the external device, the tubular
enclosure extending in the axial direction beyond the end of the
conductor sections by a predetermined amount so as to form a
circular waveguide antenna with a periodic wave propagation, the
coupling elements of which are the conductor sections, and
reflectors terminating the other ends of the conductors to reflect
waves of the high-frequency field so that a high-frequency field
oscillating in phase is formed by the tubular enclosure and the
conductor sections.
Inventors: |
Krause; Norbert (Heroldsbach,
DE) |
Assignee: |
Siemens Aktiengesellschaft
(Munich, DE)
|
Family
ID: |
6231073 |
Appl.
No.: |
06/713,635 |
Filed: |
March 19, 1985 |
Foreign Application Priority Data
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Mar 20, 1984 [DE] |
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3410204 |
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Current U.S.
Class: |
324/318 |
Current CPC
Class: |
G01R
33/3453 (20130101) |
Current International
Class: |
G01R
33/34 (20060101); G01R 33/345 (20060101); G01B
033/20 () |
Field of
Search: |
;324/309,318-320 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
"Research Papers", Resolution and Signal-to-Noise Relationships in
NMR Imaging in the Human Body, J. Phys. E: Sci. Instrum., vol. 13,
1980, printed in Great Britain, J. M. Libove and J. R. Singer, May
2, 1979, pp. 38-44. .
D. Homogene Wellenleiter, Insbesondere Hohlleiter, Taschenbuch der
Hochfrequenztechnik, H. Meinke and F. W. Gundlach, 1968, pp.
308-317, 332-339, and 462-465..
|
Primary Examiner: Tokar; Michael J.
Assistant Examiner: Jaworski; Francis J.
Attorney, Agent or Firm: Kenyon & Kenyon
Claims
What is claimed is:
1. An antenna device for generating a high-frequency magnetic field
in an apparatus for nuclear spin tomography, comprising:
(a) an energy source;
(b) at least two conductor sections of predetermined length which
extend on an imaginary cylinder surface parallel to the direction
of the axis of the imaginary cylinder and are arranged for current
flow in opposite directions when one end of each of said sections
is connected to said energy source;
(c) a tubular enclosure arranged at a predetermined distance
concentrically with respect to the imaginary cylinder surface,
surrounding the conductor sections, said enclosure being at least
largely permeable to low frequencies for magnetic gradient fields,
containing electrically highly conductive material and connected to
said energy source, said tubular enclosure extending in the axial
direction beyond the end of the conductor sections by a
predetermined amount to form a circular waveguide antenna with
aperiodic wave propagation in a cut-off mode at the axial ends of
said enclosure, the coupling elements of which are said conductor
sections; and
(d) means for reflecting the waves of the high-frequency field
terminating the other ends of said conductors so that a
high-frequency field oscillating in phase is formed by the tubular
enclosure and the conductor sections which field is attenuated
axially beyond said conductor sections.
2. An antenna device according to claim 1, wherein the axial length
of said tubular enclosure forming a circular waveguide antenna is
at least twice as large as the corresponding length of the
conductor sections.
3. An antenna device according to claim 1, wherein said individual
conductor sections are arranged symmetrically with respect to the
axial center of said tubular enclosure serving as the circular
waveguide antenna.
4. An antenna device according to claim 1, wherein said tubular
enclosure forming a circular waveguide antenna comprises a foil or
film of electrically highly conductive material which is applied to
the inside or outside of a carrier body consisting of electrically
insulating material.
5. An antenna device according to claim 4, wherein the thickness of
said part of the tubular enclosure which forms the circular
waveguide antenna consists of electrically conductive material is
between 10 and 50 .mu.m.
6. An antenna device according to claim 5, and further including a
further system of coupling elements disposed at at least two
parallel cross-sectional planes which are symmetrical to the
central cross-sectional plane passing through said tubular
enclosure forming a circular waveguide antenna.
7. An antenna device according to claim 1, wherein the thickness of
the part of said tubular enclosure which forms the circular
waveguide antenna consisting of electrically conductive material is
between 10 and 50 .mu.m.
8. An antenna device according to claim 1, and further including a
further system of coupling elements disposed at at least two
parallel cross-sectional planes which are symmetrical to the
central cross-sectional plane passing through said tubular
enclosure forming a circular waveguide antenna to further attenuate
the magnetic field in the axial direction beyond said conductor
sections.
9. An antenna device according to claim 8, wherein said further
system of coupling elements is disposed in the region of the end
faces of the conductor sections.
10. An antenna device according to claim 9, wherein the coupling
elements are bracket-shaped and have axially extending conductor
sections which are connected on one side to the electrically
conductive material of said tubular enclosure forming a circular
waveguide antenna in an electrically conducting manner.
11. An antenna device according to claim 10, wherein in said
further system the flow directions of the currents are opposed to
each other in the axially extending conductor sections of the
corresponding coupling elements.
12. An antenna device according to claim 9, wherein each further
coupling system comprises at least one pair of diametrically
opposite coupling elements which are arranged on the inside of said
tubular enclosure forming a circular waveguide antenna.
13. An antenna device according to claim 12, wherein the coupling
elements are bracket-shaped and have axially extending conductor
sections which are connected on one side to the electrically
conductive material of said tubular enclosure forming a circular
waveguide antenna in an electrically conducting manner.
14. An antenna device according to claim 13, wherein in said
further system the flow directions of the currents are opposed to
each other in the axially extending conductor sections of the
corresponding coupling elements.
15. An antenna device according to claim 8, wherein each further
coupling system comprises at least one pair of diametrically
opposite coupling elements which are arranged on the inside of said
tubular enclosure forming a circular waveguide antenna.
16. An antenna device according to claim 15, wherein the coupling
elements are bracket-shaped and have axially extending conductor
sections which are connected on one side to the electrically
conductive material of said tubular enclosure forming a circular
waveguide antenna in an electrically conducting manner.
17. An antenna device according to claim 16, wherein in said
further system the flow directions of the currents are opposed to
each other in the axially extending conductor sections of the
corresponding coupling elements.
18. An antenna device according to claim 8, wherein the coupling
elements are bracket-shaped and have axially extending conductor
sections which are connected on one side to the electrically
conductive material of said tubular enclosure forming a circular
waveguide antenna in an electrically conducting manner.
19. An antenna device according to claim 18, wherein in said
further system the flow directions of the currents are opposed to
each other in the axially extending conductor sections of the
corresponding coupling elements.
20. A method of operating an antenna device which antenna
comprises:
(a) at least two conductor sections of predetermined length which
extend on an imaginary cylinder surface parallel to the direction
of the axis of the imaginary cylinder and are arranged for current
flow in opposite directions when one end of each of said sections
is connected to an external current source;
(b) a tubular enclosure arranged at a predetermined distance
concentrically with respect to the imaginary cylinder surface,
surrounding the conductor sections, said enclosure being at least
largely permeable to low frequencies for magnetic gradient fields,
containing electrically highly conductive material and connected to
said current source, said tubular enclosure extending in the axial
direction beyond the end of the conductor sections by a
predetermined amount so as to form a circular waveguide antenna
with a periodic wave propogation and operating in a cut-off mode at
the axial ends of said enclosures, the coupling elements of which
are said conductor sections;
(c) mcans for reflecting the waves of the high-frequency field
terminating the other ends of said conductors so that a
high-frequency field oscillating in phase is formed by the tubular
enclosure and the conductor sections; and
(d) a further system of coupling elements disposed at at least two
parallel cross-sectional planes which are symmetrical to the
central cross-sectional plane passing through said tubular
enclosure forming a circular waveguid antenna, said method
comprising the steps of:
adjusting the phase and amplitude conditions at the additional
coupling systems by means of an energy feeding device connected
thereto in such a manner that the field strengths caused by these
coupling systems together with the field strength by the conductor
sections alone are superimposed to form a total field strength
which is practically zero outside of the region bounded by the
parallel cross-sectional planes of the two additional coupling
systems.
Description
BACKGROUND OF THE INVENTION
This invention relates to nuclear spin tomography in general and
more particularly to an antenna device for exciting an at least
largely homogeneous magnetic high-frequency field and/or for
receiving corresponding high-frequency signals in a nuclear spin
tomography apparatus.
An antenna device for exciting an at least largely homogeneous
magnetic high-frequency field and/or for receiving corresponding
high-frequency signals in apparatus for nuclear spin tomography, in
which device at least two conductor sections of predetermined
length are provided which extend on an imaginary cylinder surface
parallel to the direction of the cylinder axis, through which
current flows in opposite directions and which are connected to an
external energy feeding device is disclosed in DE-OS No. 3133432.
The antenna device further contains a tubular enclosure which is
arranged at a predetermined distance concentrically with respect to
the imagined cylinder surface and around the conductor sections.
The enclosure passes at least largely, low-frequency magnetic
gradient fields, contains electrically highly conducting material
and is likewise connected to the energy feeding or receiving
device. The conductor sections are terminated at their respective
end which is not connected to the energy feed or receiving device
by means reflecting the waves of the high-frequency field so that a
high-frequency field which oscillates in phase can be developed by
the conductor system formed by the tubular enclosure and the
conductor sections.
In the field of medical diagnostics image-forming methods have been
developed in which resonance signals integrated by calculation or
measurement of nuclei of a given element of, in particular, a human
body or part of a body are analyzed. From the spatial spin density
and/or relaxation time distribution so obtained, an image similar
to an x-ray tomogram can be constructed. Such methods are known
under the designation "Nuclear Spin Tomography" (Nuclear Magnetic
resonance tomography) or "Zeugmatography."
A requirement in nuclear spin tomography is a strong magnetic field
which is generated by a so-called base field magnet, is as
homogeneous as possible in a region of predetermined extent and
into which the body to be examined is placed along an axis which
generally coincides with the orientation axis of the magnetic base
field. Superimposed on this base field are stationary and/or
pulsed, so-called gradient fields. For exciting the individual
atomic nuclei in the body to perform a precession motion, a special
antenna device is further required, by which means of a
high-frequency magnetic alternating field (RF alternating field)
can be excited for a short time and which can also be used for
receiving the RF signals connected thereto if a separate measuring
coil is not provided for this purpose.
As is well known, the quality of the sectional images in such
apparatus for nuclear spin tomography (NMR tomography) depends on
the signal-to-noise ratio of the induced nuclear spin resonance
signal. Since this signal-to-noise ratio in turn depends on the
strength of the magnetic base field and increases with frequency,
it is desirable to provide frequencies as high as possible for high
base fields (see "Jour. Phys. E: Sci. Instrum.", volume 13, 1980
pages 38 to 44).
With the known RF antenna device mentioned above, RF fields with
high frequencies of about 20 MHz or more can be excited and
received. To this end, the antenna device contains a tubular
antenna part of electrically highly conductive nonmagnetic
material. This antenna part represents and envelope around several
conductor sections which form at least one pair of conductors which
are disposed on an imaginary cylinder surface, around which the
envelope is arranged concentrically at predetermined spacings. On
the at least one conductor pair and the envelope and wave
propagation with very high frequency is then made possible,
resonance conditions being adjusted in such a manner that fields
oscillating in the same phase are developed in the entire volume of
interest in the form of standing waves on the pair of conductors.
Since, furthermore, the common envelope around the pair of
conductors is designed so that it passes, at least largely, low
frequencies, the low-frequency gradient fields can accordingly
propagate unimpeded in the volume into which the body to be
examined is to be placed.
In this known antenna device, however, the alternating RF field can
also cover regions which are located in front of the respective
axial end faces of the conductor system formed by the conductor
sections and the tubular enclosure. This means that disturbances
caused in these regions can possibly falsify the high-frequency
measuring signal.
It is, therefore, an object of the present invention to improve
this known antenna device in such a manner that it is largely
independent of external interference fields.
SUMMARY OF THE INVENTION
According to the present invention, this problem is solved by
extending the tubular enclosure in the axial direction beyond the
ends of the conductor sections at the end face by a predetermined
amount, so that a circular waveguide antenna with aperiodic wave
propagation is developed by it, the coupling elements of which are
the conductor sections.
In this antenna device, high-frequency power is coupled in the
tubular enclosure via conductor sections which are located in the
region of the axial center of the tubular envelope serving as the
circular waveguide, where an aperiodic wave propagation is set due
to the predetermined dimensions of the tubular enclosure (see, for
instance, H. Meinke/F. W. Grundlach: Taschenbuch der
Hochfrequenztechnik, 3. Edition, 1968, pages 309 to 316). Since the
diameter of the tubular enclosure is always small relative to the
wavelength of the high-frequency field of 20 MHz or higher, this
circular waveguide antenna can be operated in a so-called "cut-off"
region below a given critical frequency, in that the propagation of
the high-frequency field is limited practically completely to the
space enclosed by the tubular enclosure. The advantages connected
with this embodiment of the antenna device are then seen in the
fact that in this manner an interaction of the high-frequency field
with external interference fields can be prevented at least to a
high degree.
According to a further embodiment of the antenna device of the
present invention, an additional coupling system may be provided
advantageously in at least two parallel cross-sectional planes
which are symmetrical to the central cross-sectional plane through
the tubular enclosure serving as the circular waveguide antenna.
Such an antenna device is advantageously operated in such a manner
that the phase conditions and the amplitude conditions at the
additional coupling systems are set, by means of the energy feeding
device connected to them, in such a manner that the field strengths
caused by the coupling system formed by the conductor sections and
by the additional coupling systems alone are superimposed to form
an overall field strength which is practically zero outside the
region bounded by the parallel cross-sectional planes of the two
additional coupling systems.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal section of a high-frequency antenna device
according to the present invention indicated schematically.
FIG. 2 is a transverse cross-section of the antenna of FIG. 1.
FIG. 3 is a longitudinal cross-section of a further embodiment of
an antenna device according to the present invention indicated
schematically.
FIG. 4 is a diagram illustrating the field strength conditions
which can be developed with the antenna of FIG. 3.
DETAILED DESCRIPTION
The high-frequency antenna device according to the present
invention is to be provided for apparatus in nuclear spin
tomography known per se. Such apparatus comprises in general at
least one normal or, in particular, superconducting field coil
system which is arranged concentrically with respect to the z-axis
of an x-y-z coordinate system and with which a strong base field as
homogeneous as possible is produced. In addition, gradient coils
for generating sufficiently constant magnetic field gradients are
provided. The magnet coils permit axial access to the homogeneous
field region at their center, i.e., in particular, a human body or
body part to be examined is placed in the magnetic field along the
z axis. The nuclear spin is excited by means of an RF field as
homogeneous as possible which is oriented perpendicular to the z
axis.
An embodiment of a high-frequency antenna device according to the
present invention which can be used for this purpose, is shown in
FIG. 1 as a longitudinal section and in FIG. 2 as a cross section.
In the Figures, like parts are provided with the same reference
symbols. The antenna device, in which the device shown in DE-OS No.
31 33 432 is taken as the basis, contains a hollow cylindrical
tubular enclosure 2, the cylinder axis of which points in the
direction of the z axis of an orthonogal x-y-z coordinate system.
The coordinate origin O is placed in the axial center of the
tubular envelope. This tubular enclosure with an axial length L and
a diameter D consists of an electrically highly conductive
nonmagnetic material such as copper and may optionally be
silverplated at least on one side. The tubular enclosure 2 is
formed, for instance, by a nonmetallic, electrically insulated
tubular carrier 3, to the inside of which a thin copper foil or
film is applied. According to the illustrated embodiment assumed in
FIGS. 1 and 2, the tubular enclosure itself is therefore formed by
this copper foil 4. Optionally, the copper foil can also be
arranged on the outside of a corresponding carrier tube. Since the
tubular enclosure should be sufficiently permeable for the
low-frequency gradient fields, the wall thickness d of its
electrically conductive material must, on the one hand, be
relatively small. On the other hand, the RF resistance of the
tubular envelope increases with decreasing thickness, for which
reason the thickness d should advantageously be chosen larger than
the depth of penetration of the high-frequency field. In general,
the thickness d to be provided is between 10 and 50 .mu.m.
The tubular enclosure 2 encloses several electric conductors lying
on at least one imaginary cylinder surface. The distance a between
the tubular enclosure 2 and the cylinder surface which is indicated
by a dotted line 6 and is arranged concentrically therewith, has a
predetermined value a. At least two conductor sections 7 and 8 can
be provided which form a conductor pair, have an axial length s and
through which a current I flows. Advantageously, a respectively
predetermined number of parallel-connected conductor sections is
used which lie side-by-side with spacing on the cylinder surface 6
(see the cited DE-OS No. 31 33 432).
With this system formed by the tubular enclosure 2 and the
conductor sections 7 and 8, a high-frequency magnetic field H.sub.1
is generated (FIG. 2) which is indicated by lines 9 with arrows and
is largely homogeneous, at least about the central examination
region around the coordinate origin O and is oriented
perpendicularly to the z axis. For this purpose, a high-frequency
current is fed from an energy feeding device, which is only
indicated in FIG. 1, and comprises a generator 11, amplifiers 12
and 13 as well as matching tranformers 14 and 15, into the
conductor sections 7 and 8 as well as into the tubular enclosure 2
in such a manner that standing waves are formed in this system of
conductor sections and the tubular envelope, the operation being at
resonance. As is indicated by the current arrows at the leads 17
and 18 leading to the conductor sections 7 and 8, the current I in
the diametrically opposite conductor sections 7 and 8 is to flow in
opposite directions. In the antenna volume of interest, high
frequency fields oscillating in phase are thus generated. In order
to limit the length s of the conductor sections 7 and 8 required
for resonance operation, capacitances 21 and 22 with predetermined
values are provided in a manner known per se, for instance, at
their ends, between the tubular enclosure 2 and the conductor
sections 7 and 8 (see the cited DE-OS No. 31 33 432).
In order to shield the system formed by the conductor sections 7
and 8 and the tubular enclosure 2 surrounding them against
interfering radiation from the end face, the length L of the
tubular enclosure 2 is at least twice as large as the length s of
the conductor sections. The tubular enclosure 2 and the conductor
sections 7 and 8 extend symmetrically to the cross-sectional plane
(z=0) extending through the coordinate origin O. For, with these
measures and due to the fact that the diameter D of the tubular
enclosure 2 is small relative to the wavelength of the RF field,
the tubular enclosure 2 will act like a circular waveguide antenna
which is operated below a given critical frequency in a so-called
"cutoff" range mode. The conductor sections 7 and 8 then represent,
in the region of the cross-sectional plane extending through the
center of its axial dimension, a system of coupling elements for
coupling in and out the corresponding high-frequency power. If,
therefore, high-frequency power is fed in the plane z=0 of the
tubular envelope extending from z=-L/2 to z=+L/2, the magnetic RF
field drops off toward positive and negative z according to a
function exp (-.alpha..vertline.z.vertline.) (see the mentioned
Handbook, page 309) Alpha is a frequency-dependent numerical value
typical in the tubular enclosure for the state of the field, the
so-called mode. Particularly advantageous for application in
nuclear spin tomography is operation in the so-called H.sub.11 mode
with a magnetic field H.sub.1 transverse to the z axis, as can also
be seen in FIG. 2 (see, for instance, the cited Handbook, pages 332
to 334).
According to the embodiment of FIGS. 1 and 2, it was assumed that
an excitation of the tubular enclosure 2 acting as a circular
waveguide antenna takes place only in the transverse center plane
(z=0) via a corresponding coupling system formed by the individual
conductor sections. However, it is particularly advantageous if the
excitation takes place in several transversal planes, where further
coupling systems are provided in tranversal planes lying
symmetrically to the transversal center plane (z=0). Thus, the
excitation may take place particularly in three transversal planes.
Such an excitation system is schematically illustrated in FIG. 3 in
a longitudinal section through the antenna device according to the
present invention. Parts coinciding with FIG. 1 are provided with
the same reference symbols.
The antenna device according to the present invention shown in FIG.
3 contains, besides the at least one pair of conductor sections 7
and 8, in the region of the cross-sectional planes terminating the
two end faces of these conductor sections (z=-s/2 and z=+s/2),
respective coupling systems 24 and 25 of at least one pair of
coupling elements 26 and 27 and 28 and 29. These coupling elements
which are arranged diametrically within a system are inductive
coupling elements known per se (see, for instance, the mentioned
Handbook, pages 462 to 465 and particularly FIG. 4.4). These
coupling elements have the shape of brackets on the inside of the
tubular enclosure 2, extend in the z direction and are connected on
one side to the electrically conducting wall, of, for instance, the
copper foil 4. Their respective other ends are brought through the
tubular enclosure 2 insulated and are connected via coaxial cables
to an external energy feeding device, not shown. The connections of
the respective coupling elements and thus, also the feedthroughs
through the tubular enclosure are arranged so that the flow
directions of the currents I' indicated by arrows at the connecting
cables in opposite coupling elements are opposite to each other as
seen in the z direction.
According to the embodiment shown in FIG. 3, the coupling systems
24 and 25 each comprise only two coupling elements 26 and 27 and 28
and 29, respectively. Optionally, however, each individual coupling
element can also be replaced by several coupling elements through
which current flows in the same direction, so that then, a
corresponding number of pairs of diametrically arranged coupling
elements is formed which carry current in opposite directions (see
the cited DE-OS No. 31 33 432).
The field strength conditions to be developed in the antenna device
according to FIG. 3 can be seen in detail from the diagram shown in
FIG. 4. The field strength which is developed at the coordinate
origin 0 and is pointing in the x direction is to have a normalized
magnitude H.sub.1 =1. In this diagram, the magnitude of this
relative field strength .vertline.H.sub.1 .vertline. is given in
the longitudinal direction of the tubular envelope as a function of
the position on the z axis. Beside the central coupling system
formed by the conductor sections 7 and 8 extending from z=-s/2 to
z=+s/2, the two coupling systems 24 and 25 are arranged at z=-s/2
and z=+s/2. The field pattern obtained in the tubular enclosure
without the use of the additional coupling systems is illustrated
by a dashdotted curve A.sub.1. This field pattern is also obtained
for the antenna device according to FIGS. 1 and 2. If, however, an
additional field is fed into the individual additional coupling
systems 24 and 25 in the regions z=-s/2 and z=+s/2, respectively,
with a defined phase and amplitude difference according to the
dotted curve A.sub.2 and A.sub.3, respectively, a very steep drop
of the field to practically zero can advantageously be obtained in
the mentioned z regions. The corresponding field pattern shown by a
solid curve A is, therefore, obtained from a superpositioning of
the field shapes A.sub.1, A.sub.2 and A.sub.3 generated by the
conductor sections 7 and 8 and the additional coupling systems 24
and 25. The field H.sub.1 is, therefore, practically zero for all z
values with .vertline.z.vertline. larger than s/2. This means that
the field is limited to a transversal region of length s. In this
manner, in particular noise pickup from parts of the body to be
examined contained in the measurement volume can be advantageously
eliminated.
According to the embodiment shown in FIG. 3 of an antenna device
according to the present invention, it was assumed that the
coupling elements 26 to 29 of the additional coupling system 24 and
25 are in the same longitudinal sectional plane through the tubular
enclosure 2 as the conductor sections 7 and 8. Optionally, however,
the coupling elements can also be arranged in longitudinal section
planes which subtend a predetermined angle with the longitudinal
section plane containing the conductor sections.
* * * * *